(a) Technical Field
The present invention relates to a method for preparing a phenolic oligomer antioxidant with a very small residual content of butylated hydroxytoluene (BHT).
(b) Background Art
In general, antioxidants added to polymers include phenolic antioxidants, amine-based antioxidants, phosphite-based antioxidant, thioester-based antioxidants, etc.
Although the phenolic antioxidants have superior thermal stability at the initial step of the oxidation process, their application is limited because they are discolored after the oxidation.
The amine-based antioxidants are also limited in their application because the materials have color themselves. The phosphite-based antioxidants are susceptible to hydrolysis, and easy to decompose during the vulcanization of the rubber. The thioester-based antioxidants are disadvantageous in that they do not show their distinct effect when used alone or together with other antioxidants.
Therefore, phenolic oligomer antioxidants are drawing attentions recently.
The phenolic oligomer antioxidant refers to an antioxidant wherein one or more monomer of diene compounds such as dicyclopentadiene (hereinafter, ‘DCPD’) is connected to the phenolic antioxidant.
The phenolic oligomer antioxidant is advantageous in that it prevents from the oxidation of polymers without negatively affecting the physical properties of the polymer. Also, because it is unharmful to the human body and has moderate volatility, it is widely used as a primary antioxidant that can be added to plastics, elastomers, mineral oils and synthetic oils.
Chinese Patent Publication No. 101402721 and U.S. Pat. No. 3,305,522 disclose, as a method for preparing the phenolic oligomer antioxidant, a method of adding an olefin to a phenolic oligomer compound, which is prepared from the reaction of a phenolic compound with DCPD thereto. However, the method according to the patent documents is problematic in that butylated hydroxytoluene (hereinafter, ‘BHT’) such as 2,6-di-tert-butyl-4-methylphenol is generated in large quantities during the preparation of the phenolic oligomer antioxidant.
Although the BHT is also used as a phenolic antioxidant, a variety of problems are reported. It can pollute the environment because it is highly volatile due to small molecular weight and the severe discoloration occurs upon its oxidation. What is worse, it is very harmful to the human body. It may be toxic to the liver and may cause allergy and tumors.
For this reason, demands on a BHT-free antioxidant are increasing rapidly.